1. Field of the Invention
The invention pertains to crystal video receivers and more particularly to crystal video receivers adaptable for detecting CW and pulsed signals and for detecting pulsed signals in the presence of CW signals.
2. Description of the Prior Art
A crystal video receiver registers a signal detection when the incident signal level exceeds a given threshold. When the threshold level is fixed the leading edge of a pulse signal crosses the threshold and locks the receiver to prevent the reception of other signals. This condition persists until the amplitude of the trailing edge decays to a level below the threshold, where after the receiver is reset for the reception of additional signals. A received CW signal having an amplitude greater than the threshold level, however, crosses the threshold and remains above it for the duration of the CW signal, thus locking the receiver and rendering it insensitive to other signals.
A CW at the receiver jammer may be frustrated with the utilization a variable threshold, such as a noise riding threshold. The threshold level of such a variable threshold is determined by the total noise in the system. CW signals are treated as noise in a noise riding threshold system and act only to increase the receiver threshold level. Thus, pulses having peak levels which exceed the amplitude of the cw signal are detectable and the receiver is operable in the presence of the received CW signal. Since the noise riding threshold treats the CW signal as noise, CW signals are not detectable and the system is only useful for the detection of pulse signals.
Direction finding systems operating in a multi-emitter environment must be capable of detecting and processing a multiplicity of received signals. Should the received signals all be pulse modulated it is essential that the video receiver in the system be impervious to lock by a received CW signal. In a multi-emitter environment wherein a CW emitter is in operation the system must be capable of detecting and processing both CW and pulse modulator signals. Such a system must be insensitive to CW signals when processing pulse signals and must have the added capability of detecting and processing CW signals.